will post when they reply. Thanks Jason.

 

Have you heard anything??😳

 

Here is the reply I had in my email this morning:

Hi Steve,

Obviously Jason is concerned about the stories we hear of broken 964 cranks. Granted the 964 crank is not on of Porsches finest pieces of work, but they have been put to the test for 25 years and are still popular. It's not what I would call a common occurrence, but needs to be put into perspective and it seems to be more frequent in racing than in other 911 race engines. You should consider all the aftermarket light weight pulleys that have been used on the 964 engine, not just our serpentine conversion kit. The most common are V-belt pulleys that were used for racing and transplanting engines into the older cars. They too were considerably lighter than the factory damper. We've been making v-belt crank pulleys for the 964 engine from the early 1990s when the engines became available. These light 964 V-belt pulleys are common and made by a number of manufacturers.

When you want a big engine in your early chassis street or race car and the flywheel and damper won't fit, they get replaced with one that will. Light weight, high performance and animalistic feel is usually acceptable/desirable on these conversions. The smooth plush feel of a new car is generally not the goal. Among club racers a stock engine with the rotating mass minimized is common. Granted this is not ideal as Porsche had that weight added to the crank for a reason. This is also where the broken crank stories begin.

We introduced our serpentine belt kit for the 964 and 993 in 2004 for track use to eliminate the stretched and broken belt issues on the race track. The serpentine belt system was most often for reliability after the damper ( approximately 10#) and dual mass flywheel ( approximately 40#) have been removed. After removing that much rotating mass, the engine becomes very crisp and responsive. If you take the dual mass flywheel and crank damper off a stock engine with the DME, the engine will not idle as well because it needs the rotating mass for the DME's programming. Replacing just the damper will have an effect on the idle, but generally not much. Another issue that arises is V-belt failures similar to the 993 because the engine's new found crispness and the belts are not rated for the load under quick RPM changes. The 9.5mm belt is only rated for about 7HP. The alternator on the 964/993 engine requires approximately 12HP for street use. The 964 has so much rotating mass it doesn't rev fast enough to toss the belt. The 993 engine having a very light crank pulley is crisper than the 964 which makes the tossed and broken belt issues on the 993 understandable. Take this a step further and add higher C/R, big cams and a different engine management system or carburetors and the harsh environment of track driving, the belt system takes quite a beating. Our serpentine belt conversion was designed to take this abuse and finish the race without losing a fan belt.

Once they started being used on street cars the kits with A/C followed in 2007 for street use. About a year ago we received a call from a shop that installed one of our early serpentine belt kits w A/C on a street car. He was replacing the tensioner and belt for the first time and reported the car had over 90K miles on our serpentine belt kit. Many of our early 964 serpentine belt systems are still on the track today and many on street cars.

Is a lighter crank pulley by itself causing broken crankshafts? For as long as light crank pulleys on 964 engines without dampers have been used, we haven't seen any evidence that a light weight crank pulley by itself will not cause a 964 crank failure.

I hope this helps put this issue into perspective.

Regards,

Richard Clewett
www.clewett.com
Richard@clewett.com
310-406-8788

 

" we haven't seen any evidence that a light weight crank pulley by itself will not cause a 964 crank failure."
Why the double negative in the last sentence?? - So he has seen ??
Wondering??

 

follow up:

thanks for the reply Richard.
so, you are saying that I can buy this product from you and install it on my stock standard 964 and have no issues?
regards
steve



Hi Steve,



I’m a firm believer in never saying never because Murphy is alive. With a stock standard 964 you should have no issues. Sorry about the non-absolute answer.



That said, there is one occasional issue that pops up and that is low charging voltage at idle. Your stock pulley system runs the fan and alternator at different speeds. Obviously that’s not something we can do with a single belt. Our kit is very close to matching the fan speed which slows the alternator. Normally the charging light will go out and stay out. If you should encounter the charging light staying on at idle (“issue”), you’ll need to install small light in parallel and similar size to the light in the dash. We typically recommend putting the light in the engine compartment. It goes on when the ignition is turned on and out as soon as the engine is started just like the light in the dash. This light pulls a little more current through the alternator field coils and helps the alternator charge at a lower engine speed.



Regards,



Richard Clewett

www.clewett.com

Richard@clewett.com

310-406-8788

 

and further follow up:

thank you Richard for your reply. I understand Murphy is a worldwide concern I was more worried about Jason's comments in-so-far as the pulley (dampner) making things 'come loose' over time. Do you have customers that have installed this on stock 964 (with A/C) and have put many miles (years) on it since ?
I really like the product, but when I read other 911 users comments to the contrary, clearly I am cautious.
regards
steve


Your welcome Steve.



Comments like Jason’s will certainly raise a lot of questions. We’ve been producing the kits with A/C since 07 and have been working flawlessly. I think it is safe to say that all of the kits with A/C are on street cars. How many are driven daily is hard to know. My guess is probably 30% are daily drivers. Our customers that use this kit like it. The shops that install them come back for more.



I’m not sure why Jason says things come loose over time. What things? The 964 engine with the 40# flywheel is a smooth running engine even without the damper. Follow the installation instructions and you won’t have any problems http://www.clewett.com/instrux/36serp.pdf



It good that there are people like you that look for real information and common since. I understand your caution. The internet is full of GI/GO (garbage in / garbage out) comments. It can be difficult sometimes to filter through the garbage and get to the good stuff. Our kits have been on the market long enough that if there were a problem, you would know it.



Regards,



Richard Clewett

www.clewett.com

Richard@clewett.com

310-406-8788

 

The Porsche 962 use to snap crankshafts because of this problem. The 997 GT3 when running a LWF without a harmonic dampener (there is an aftermarket version available) suffers from loose cam actuator bolts, flywheel bolts, crank pulley bolts, etc. Here is the TSB from Porsche.

Based on their comments I am going to go out on a limb and say they never bothered testing angular vibration displacement, torsion angle orders, blah, blah, blah. The problem (torsional vibration -- the tacoma narrows bridge is a good example) has nothing to do with smoothness or balance. It sounds like maybe they weren't aware of the original problem with the 964 crankshaft? And now they aren't aware of any customers experiencing issues? Which is a good thing but it doesn't mean problems haven't occurred. If Porsche Motorsports put a harmonic balancer on the 964 cup and the 993RSR then there was probably a good reason?


Here are the original notes from Porsche about the 964 Harmonic Balancer


Here is what they had to do to remove the balancer from the 993



And here is when they raced the 993 RSR with the 964 crankshaft and had to put the 964 Turbo harmonic balancer back on the engine.



I am aware of 3 aftermarket serpentine belt style pulleys and two of those are sold with warnings that they are not to be used as replacements on a stock 964 with a worn out harmonic dampener. Not to say there haven't been any problems but would it be safe to say that the vast majority of issues with stretched and broken belts have always come down to installation error? The number of miles raced with the stock belts on (stock and modified) 964 and 993 is certainly several orders of magnitude larger than those driven with a serpentine belt although the clewett serpentine belt solution is definitely more efficient. There is no question that the clewett serpentine belt solution is better. And it solves the problem of fitting the pre-89 engine carrier to the 964 engine. The question is why would Porsche goto the expense (and weight) of putting a harmonic dampener on a very specific crankshaft if there wasn't a problem? And why would Porsche motorsport do the same thing during the years it raced the 964 cup and seven years later when they reused that crankshaft in a 993 engine?

 

A question Jason:

I friend of mine have an RSR engine in is 964 cup. He has the broader serpentine set up as I've seen on factory pictures to. I thought that this was because of two options, 1 it has another crank 2 Your supposed to tear it appart every 80th hour so it's worth the chance.

But now I see in your document above that the RSR is supposed to have a vibration damper to? Can you clarify this, if you know?

And I decided some years ago to keep the damper on my engine. If Porsche did it on the Cup cars they probably had their reasons.

 

It all depends on the crankshaft that was used and that varied with the model year. The MY97 993RSR came with a 993 crankshaft (993-102-021-70) and used a 993 style (damper-less) pulley (993-102-050-70). The earlier models with the 964 crankshaft (964-102-021-04) came with the 964 Turbo pulley & vibration damper (964-102-150-20). Both used a tensioning pulley;

 

Based on their comments I am going to go out on a limb and say they never bothered testing angular vibration displacement, torsion angle orders, blah, blah, blah.

Hi Jason,

That would be a very safe limb to climb on. The 964 crankshaft issue has been around for 20+ years. We are very aware of the shortcomings of the 964 crankshaft. You are correct; we have not bothered testing angular vibration displacement. When Porsche Motorsports has an issue, their engineering staff is all over the problem. I have no doubt that Porsche has done their homework to properly identify the problem and make appropriate corrections. The TSBs are their way of keeping us informed of problems and updates. The TSB above is referring to is regarding the GT3 (997) crankshaft and replacing the dual mass flywheel with a single mass flywheel is valid. It’s not about 964 damper. This warning is for a different crankshaft and application although the 964 problem is 10 years older and similar. Porsche must put out these TSB warnings to keep everyone informed. They are stating what could happen not necessarily that it will. It is a warning.

Did Porsche know what they were doing when they put the damper in the crank pulley? Absolutely, YES! There is no doubt in my mind that Porsche put dampers and dual mass flywheels on the engines for a reason.

If you have concerns about things coming loose and crankshaft failures from not having a damper in the crank pulley, you should purchase the factory belt tensioning tool, belts and use only factory equipment. Hopefully your 20+ year old damper is still in good balance and condition. They do need to be replaced at times and they are often overlooked. Just like fan belts and hoses, rubber products harden with heat and age. The A/C belt is applying a constant side load on the rubber damper with the engine stopped or running. It’s questionable that a 20 year old damper is working as well as originally designed.

As I understood Steve he has a stock 964 street car. I’m assuming that your dual mass flywheel is still in place and that this is a street driven car and that you are not participating in any 24 hour races. If this is the case our serpentine conversion kit is a great alternative.
Our serpentine conversion kit has been used for over 10 years providing reliable service and no broken crankshafts.

Richard

 

We don't know the full spec of my friends engine. It is a 1990 cup that was modified to 964 RSR with factory parts for the season 1993. So I guess it still has a 964 crank, of some sort.

 

So we probably shouldn't be replacing a solution to a real problem with a part that ignores it?


Exactly, the 997GT3 TSB was a recent example of the problem occuring to a Porsche that didn't have a harmonic dampener, ie. this is what can happen. It doesn't happen to everybody but it happens. And there are now aftermarket harmonic dampeners available for that specific problem.


The issue is resonance, so what matters is that we are not driving at a sustained RPM for a certain period of time? And you and I don't know what that RPM range is. Or any of the parameters of this issue for that matter. So neither of us can say its safe or not. If we can't define the problem how can we offer a solution? We are stuck relying on Porsche's engineering for this narrow (964) application (for the 993 your product is awesome). And what I tried to point out (with the motorsports documentation) is that when PMS had multiple opportunities to remove a heavy harmonic dampener in a power-is-everything / cost-is-no-object setting (993RSR), they still used it.

 

Jason,
The 964 crank pulley damper debate has been going on for 20+ years. Everyone is entitled to an opinion.
We appreciate the kind feedback on the 993 serpentine conversion. Thank you.
The TSB that you are referencing is specifically for the dual mass flywheel on the GT3. If you are on the track with a GT3 at sustained track conditions and speeds, you need to take the TSB seriously.

 

open question hopefully someone with some first hand expereince can answer

Has anyone on here actually had a 964 crank shaft fail due to fatigue caused by NOT running a torsional vibration damper pulley?

I like to deal with facts so I'm not interested in what people have read on the internet but only interested to hear of experiences owners and tuners have seen with their own eyes both through racing and road use.

Also got a question to Richard at Clewett, what is the weight differenece between your system and the stock 964 no aircon assembly?

 

I recently had the serpentine belt added to my engine but cannot offer any feedback, because the car is still being prepped, however, the belt was recommended by the shop as my harmonic balancer was out of spec.
biggles